<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .day {
            width: 300px;
            display: flex;
        }
        
        .day div {
            width: 50px;
            height: 50px;
            background-color: #333;
            color: white;
            line-height: 50px;
            text-align: center;
            font-size: 18px;
            margin-right: 20px;
        }
    </style>
</head>

<body>
    <div class="day">
        <div class="hour">1</div>
        <div class="minute">2</div>
        <div class="second">3</div>
    </div>
    <script>
        // 1.获取元素
        var hour = document.querySelector('.hour');
        var minute = document.querySelector('.minute');
        var second = document.querySelector('.second');
        var inputTime = +new Date('2021-06-29 17:00:10');
        countDown(); //先调用一次这个函数，防止第一次刷新页面有空白
        // 2.开启定时器
        setInterval(countDown, 1000);

        function countDown() {
            var nowTime = +new Date(); //返回当前时间总的毫秒数
            var times = (inputTime - nowTime) / 1000; //times是剩余时间总的毫秒数

            var h = parseInt(times / 60 / 60 % 24);
            h = h < 10 ? '0' + h : h;
            hour.innerHTML = h;

            var m = parseInt(times / 60 % 60);
            m = m < 10 ? '0' + m : m;
            minute.innerHTML = m;

            var s = parseInt(times % 60);
            s = s < 10 ? '0' + s : s;
            second.innerHTML = s;
        }
    </script>
</body>

</html>